Acetylene blacks are inter alia used as electrically conductive agents in a polymeric matrix. Acetylene blacks are obtained in production in form of fine powders and are therefore in general pelleted for ease of handling and shipping. Thus, one requirement of acetylene black pellets is that they are sufficiently mechanically stable to withstand break-up and attrition of the pellets during handling and shipping resulting in undesired fines. On the other hand it is important for the end users that the acetylene black pellets can be easily and homogeneously dispersed in the polymeric matrix without formation of large agglomerates of acetylene black resulting in undesired defect areas in the final product. Thus, the mechanical strength of acetylene black pellets cannot be increased to an extent that would jeopardize the dispersion quality of the acetylene black in the polymeric matrix of the final product. These two important requirements for acetylene black are difficult to achieve simultaneously and consequently there have been numerous attempts in the industry in the past to produce acetylene black pellets that at the same time withstand substantially break-up and attrition during handling and shipping and still can be easily dispersed in the polymeric matrix resulting in a homogeneous distribution of the carbon black in the matrix with a minimum of defect areas.
In DE 35 12 479 the problem of the above discussed balance of properties of pelleted acetylene black is discussed and it is proposed to provide acetylene black pellets having an individual pellet hardness of less than 5 g per pellet to ensure the required dispersibility of the acetylene black pellets in the polymeric matrix. Although it is indicated in this reference that the size of the acetylene black pellets can be varied in a broad range of 0.5 to 5 mm, it is evident from the experimental data when comparing two size fractions of the acetylene black material that the dispersibility of the size fraction of 2 to 3.2 mm is considerably deteriorated compared to the size fraction of 0.1 to 2 mm. This is clearly shown by the reduction of the impact strength of the polymeric material including the acetylene black and the considerable increase of the number of large aggregate per defined area. Thus it is evident from DE 35 12 479 for a person skilled in the art that large pellet sizes are to avoided if good dispersibility of the acetylene black in the polymeric matrix is required.
According to EP A 785 239 filed by the same applicant as DE 35 12 479 the problem of mechanical stability and dispersibility of acetylene black pellets is discussed. According to the teaching of that reference it is important in contradiction to DE 35 12 479 that the individual pellet strength is more than 5 g per pellet in order to avoid fine formation from handling and to improve the dispersibility of the acetylene black in the polymeric matrix. According to the teaching of EP A 785 239 this can be achieved by a two-step pelletizing process wherein a soft core of acetylene black is coated with a hard acetylene black shell resulting in a core/shell structure. As it is particularly evident from comparing comparative example 4 of this reference with example 3 that differ only in that in comparative example 4 the second process step resulting in the hard shell has been omitted, not only the content of fines has been increased considerably compared to example 3 what would have been expected, but also the dispersibility of the softer core material is reduced compared to the core/shell material as is shown by the increase of volume resistivity.
A similar concept of core/shell pellets was proposed in JP 3681253 and JP 3681266.
Although these references show that the formation of core/shell pellets may lead to an improved balance of mechanical strength and dispersibility of acetylene black pellets this technology has the disadvantage that the production process is rather complicated with increased energy consumption, investment and production costs versus a one-step process. Thus, it still would be desirable to obtain acetylene black pellets without the necessity of core/shell structures and thus a two-step process but still having an optimum balance of mechanical strength and dispersibility.
Another approach to form non-core/shell pellets was proposed in EP A 2 075 291. According to the teaching of this reference it is essential to select granulated acetylene black to have an average aspect ratio of at most 1.1, an average maximum pellet size from 0.1 mm to 1 mm and an average pellet size of from 0.2 to 0.6 mm. Thus, EPA 2 075 291 confirms the conclusion from DE 35 12 479 that individual pellet strength is a function of pellet size and increases with pellet size with the result that pellet size of the pellets should be low, i.e. within the range of 0.2 to 0.6 mm in order to achieve the required dispersibility. Particularly from comparative example 7 in EP 2 075 291 having an average pellet size of 0.75 mm, but still the required aspect ratio the individual pellet hardness is increased to a value of 5.5 g per pellet, thus above the limit as taught in DE 35 12 479. This results in a reduction of pulverization but also in an increase of volume resistivity and number of hard spots indicating a considerably reduced dispersibility.
Furthermore, pelleted acetylene black products are on market for example the product Denka Black Grade Granular available from DENKI KAGAKU KOGYO KABUSHIKI KAISHA, Japan. The properties of this commercial product are shown in the experimental part of present application. Particularly this product has an average pellet size of 0.7 mm.
As evident from the discussion of the prior art there is still a need in the industry to have pelleted acetylene black material exhibiting an optimum balance of attrition stability and dispersibility in polymeric matrices that can be produced in a cost-effective process. Furthermore for the end-users of these acetylene black pellets it would be advantageous if not only the required dispersibility is provided but also the energy required for dispersing the acetylene black pellets can be reduced.
Thus, it is an object of the present invention to provide pelleted acetylene blacks that lead to a reduction of the energy required for dispersing the acetylene black in polymeric matrices without compromising handling properties and dispersibility. According to a further preferred aspect of the present invention it is advantageous if the dispersibility is further improved.